It is the nature of operations in the Kraft pulping industry to produce many by-product streams en route to producing a final pulp, paper, paperboard, or linerboard product. The operating efficiency of Kraft pulping mills depends heavily on the efficient operation of a processing chemical regeneration cycle, or Recovery Cycle. In this area, maintenance of a proper balance of, for example, specific chemicals in specific by-product streams, or a particular range of solids in such streams is required for efficient mill operation.
At the very beginning of the papermaking process, in a paper mill employing Kraft pulping technology, is the digester. Simply put, the digester accepts wood chips, and converts the same into usable cellulose fibers. This is done through a predetermined application of high temperatures and pressures, and various chemicals (for example, sodium hydroxide) in a water based solution. The primary waste stream from the digester is the pulp stream (washed cellulose fibers in a water suspension), and the secondary, or waste stream, is the black liquor stream, which is an aqueous stream containing the spent processing chemicals and various by-products produced by the Kraft Digestion process.
Black liquor leaving the digestion (or pulp production) area of the process is referred to as Weak Black Liquor, and typically contains various dissolved organic and inorganic solids in the range of 12-18 percent by weight. At this dissolved solids content (particularly at the upper end of this range) a by-product precipitate forms which is called Crude Tall Oil Soap. This soap has a bulk density which is typically lower than that of the Black Liquor at this point in the Recovery Process. The soap density range can be between approximately 0.5 to 9.0 pounds per gallon, dependent upon the amount of air and black liquor contained in the soap mass. Soap of this type, having a bulk density of less than 1 pound per gallon, is considered to be foam, or very light soap.
The Weak Black Liquor and precipitated Crude Tall Oil Soap mixture is typically sent to an interim Weak Black Liquor Storage Tank, of sufficient volume to allow for reasonable process flow variations. These tanks are typically rather large (100,000 to 1,000,000 Gallons), and the rather static conditions that exist in them allow the precipitated soap to separate from the black liquor, and rise to the surface of the black liquor in the tank, thus forming a layer of soap on top of the liquor. This soap layer can be several feet thick in a typical Weak Black Liquor Storage Tank.
When soap rises to the top of the black liquor in the Weak Black Liquor Storage Tank, a natural density gradient occurs within the soap mass. This gradient is typically proportional to the percentages of air and black liquor contained in the soap, producing a light soap at the top of the soap mass, and a heavier soap at the soap/liquor interface. The density, nature and consistency of soap will vary depending upon the percentages of air and black liquor entrained in the soap. For example, when soap is heavily laden with black liquor, it becomes more like black liquor in terms of its flow characteristics. Conversely, when soap is relatively free of black liquor (for example 2-5% by weight, black liquor) the soap become viscous, sticky, sometimes lumpy and difficult to move. As a result, soap with low black liquor content may clog pipes, valves and soap recovery or removal equipment.
Soap masses in Weak Black Liquor Storage Tanks can become so large, that they represent a processing hazard, or deterrent, in several ways. A large, relatively low density, soap mass can limit black liquor storage capacity, which limits the flexibility of operation, and usually results in environmentally hazardous soap spills. If the soap mass in the tank is allowed to deaerate, and become as dense as the parent Weak Black Liquor in the tank, the soap will sink into the liquor mass, and enter the subsequent evaporation operation. Soap in liquor fed to Black Liquor Evaporators, will completely disrupt the evaporator operation. It is imperative, therefore, that the by-product soap be removed from the Weak Black Liquor Stream as efficiently as possible, before reaching the evaporators. The largest volume of soap can be separated most efficiently from the Weak Black Liquor stream at the Weak Black Liquor Storage Tank system. Separation or recovery of the soap at this point has to date been severely hindered by a lack of equipment and/or techniques to do the job satisfactorily.